55 calibre meaning 55 x diameter of the round, not 5.5 inches. I think earlier marks were 4.7 inch as opposed to 4.5 inch. There is a marvelous video in you tube of a 4.5 in action, the loading/firing mechanism is a thing to behold!

Well Nick came the nearest however the calabre of the guns in question was 4.5 inch not 5.5 inch as stated.

I was looking for ships that are currently afloat - perhaps I did not make that clear but here goes:-

Part1
The only current ship afloat in the UK with a 4.5 inch Mark 5 is HMS Calavlier part of the Chatham Historic ship collection.
There are currently no UK Ships afloat with a Mark 6 turret, however, the Austrialians have a Daring Class Destroyer still afloat as a museum ship HMAS Vampire
The current naval gun on RN Ships fitted with a gun is the 4.5 inch Mark 8 but watch this space with the new generation of ships now being built.

Part2
The odd one out is :-
Otto Melara 76mm Naval Gun not a Brisish Gun or Calibre but watch this space.
GPMG are in common use for close range work on most of the larger RN ships including submarines when they are on the surface.

Mk8, Kryten's head, found on Type 23, one piece ammunition, 55 calibre, un manned fully automatic turret. Mk 6, stop gap between Mk 5 and mk 8, Rothsey Class frigates, (eg, HMS Plymouth) hand fed, two piece amunition. Mk 6 refers to the mounting, manned turret, semi automatic. Mk 5 the gun proper. 45 calibre. Mk 5, quick firing high angle, two piece amunition, 45 calibre, hand fed with rammer and loading tray, manned turret, HMS Cavalier.
Odd man out? I would say the GPMG, as it is the only portable weapon.